The following prior art documents are believed to summarize the state of the art:
U.S. Pat. Nos. 5.982.921; 4,579,455; 4,969,198; 4,805,123; 5,185,812; 6,818,459; 6,816.249; 6.282.309; 6.081.325; 6,078,386; 6,020,957; 5.917,588; 5,825,482; 4,845,558; 4.579,455; and 4.532.650, and published U.S. Patent Application US20040066507A1.